The present invention relates to a method of detecting flaws in a thick wall steel pipe with an ultrasonic angle beam technique.
As is well known, ultrasonic waves used for detecting flaws in metal materials include a compression wave mode and a shear wave mode. A method of detecting flaws with an ultrasonic angle beam technique of shear wave mode, for example, as specified in G-0582 and Z-2344 of JIS (Japanese Industrial Standards), has been widely used to detect flaws in steel pipes.
The refraction angle of an angle probe is determined in accordance with the ratio of the wall thickness (t) to the outer diameter (D) of the pipe (hereinafter, called as t/D). However, in the case of a thick wall steel pipes having ratio t/D of 20% or more, it is difficult to detect a longitudinal flaw (a slit-like flaw parallel to the longitudinal axis of a test material) on inner surface stably by such an ultrasonic angle beam technique since the injection angle of ultrasonic wave on outer surface of pipe exceeds the critical angle related to the shear wave mode in accordance with The Law of refraction of ultrasonic wave. (This Law has been well known as Snell's Law) and the refracted shear wave does not reach the inner surface of the pipe. This phenomenon is described in "Method of Ultrasonic Examination" (revised new edition), The Nikkan Kogyo Shimbun, Co., Ltd., pages 490-493, (July 30, 1974); and "Ultrasonic Testing Method A", Japanese Society on Non-Destructive Inspection, pages 231-244, (May 1, 1974).
Various methods have been proposed for detecting a longitudinal flaw on the inner-surface of thick wall steel pipes having ratio t/D of more than 20% in the past.
FIG. 1 is an explanatory diagram illustrating a method of detecting longitudinal flaws on the inner surface of such a thick wall steel pipe. FIG. 2 is a diagram showing echo transmittance and the refraction angle of the ultrasonic waves for detecting a longitudinal flaw on inner surface of a thick wall steel pipe. Such a technique is disclosed, for example, in Japanese Patent Publication No. 17024/1981.
In FIG. 1, reference numeral 1 designates a transmitting/receiving probe for detecting a longitudinal flaw on inner surface and 2 a steel pipe to be tested having a large ratio of t/D.
In the conventional method, an ultrasonic beam injected into the steel pipe 2 (test material) at an incidence angle .alpha..sub.o in the circumferential direction from the probe 1 generates a compression wave L at a refraction angle .theta..sub.Lo and a shear wave S' at a refraction angle .theta..sub.S. The compression wave L, reflected by the outer wall of test material generates a shear wave S by mode conversion. The shear wave S reaches a point B on the inner surface of the test material at a refraction angle .theta..sub.S.
If there is a longitudinal flaw at the point B, the shear wave echo signal (shear wave mode) of the flaw travels along the same path in the reverse order of the incident wave and is mode-converted into a compression wave by refraction at the outer wall, which returns to the incident point and is received by the probe 1 as a flaw echo.
The shear wave S' at the refraction angle .theta..sub.S generated at the same time as the compression wave reaches a point A, and if there is a flaw, the wave S' returns to the probe 1 along the reverse route as a flaw echo.
In FIG. 2, T.sub.L designates a curve showing the echo transmittance of the compression wave L as a function of the incidence angle and the refraction angle .theta..sub.Lo, C an effective flaw detecting region in the method of FIG. 1 and T.sub.S a curve showing the echo transmittance of the shear wave S' generated directly at the incident point as a function of the incidence angle and the refraction angle .theta..sub.S. The echo transmittance of waves passing along the route, in the order of the compression wave L .fwdarw. the shear wave S .fwdarw. the flaw, indicated by (T.sub.L R.sub.L -S-S-L), shows that the sensitivity for detecting longitudinal flaw on inner surface of the test material is improved.
It can be said that the echo transmittance is improved with this method; however, it is also a fact that the echo transmittance is less than in the case of a shear wave refraction angle within a range of 35 to 70 degrees.
Further, because the allowable range of the refraction angle used for detecting longitudinal flaws is narrow, that is, only from 15 to 31 degrees, the range of allowable incidence angles of the ultrasonic beam onto the test material is also very narrow, that is, only from about 7 to 14 degrees, for example, in the widely used immersion method. Therefore, it is difficult to automate the testing with this method, because it is difficult to keep the accurate probe position of the testing mechanism during actual operation.
Steel pipes for a boiler or a heat exchanger, which are the main objects of applications of these methods, are rolled during manufacture, their reduction ratio is relatively small, and hence the degree of deformation in the outer and inner diameters is generally large and the pipe surfaces are generally rough. Accordingly, for inspection of such steel pipes, it is difficult to employ the above mentioned method of a longitudinal flaw detection in which flaws are detected with mode conversion by reflecting an ultrasonic beam at the outer wall of the test material because the reproductivity of flaw detection is extremely low due to scattering of the ultrasonic beam by the rough outer surface.
Moreover, if a material which has been worked is subject to flaw detection, the energy losses tend to be quite large. Accordingly, it has been strongly desired to develop a flaw detecting method which is capable of performing inspection before working on the material.